The present invention relates to a top scale balance and more particularly to a top scale balance having a Roberval's chain and a lever mechanism for load transmission. It is noted that the present invention can be applied not only to a so-called electronic balance having a load sensing portion of the electromagnetic equilibrium type, but also to a so-called scale having a load cell or the like serving as a load sensing portion.
FIGS. 9(A) and (B) are plan and side views of a conventional top scale balance, and FIG. 10 shows the mechanism thereof. In such a top scale balance, a sample scale 20 is supported through a Roberval's chain 10 (which is also called a parallel guide). This restrains the sample scale 20 from being transversely displaced and/or inclined such that the sample scale 20 is vertically moved as horizontally maintained. This prevents the occurrence of an error due to offset placing of a sample on the sample scale 20, i.e., a so-called offset position error (which is also called a four-corner error).
The Roberval's chain 10 has a structure in which a movable post 13 is connected to a stationary post 14 through two parallel horizontal members, i.e., upper and lower horizontal members 11, 12 each provided at both ends thereof with flexible portions 11a, 11b, 12a, 12b which serve as hinge portions. The sample scale 20 is supported by the movable post 13. A load acting on the sample scale 20 will be transmitted, through a lever 30 connected to the movable post 13, to a load sensing portion 40 such as a load cell, an electromagnetic equilibrium mechanism or the like.
The lever 30 is generally supported by an elastic fulcrum 31 and provided at one end thereof with a point of force 32 which is connected to the movable post 13 through a connecting piece 50. The lever 30 has the other end connected to the load sensing portion 40 such that a load to be measured acting on the sample scale 20 is to be transmitted to the load sensing portion 40. The point of force 32 of the lever 30 is flexible in the directions shown by arrows R in FIG. 10 (in the oscillating directions of the lever 30). For connection between the connecting piece 50 and the movable post 13, there is formed an elastic connection fulcrum 51. Thus, provision is made such that the elastic connection fulcrum 51 and the flexible point of force 32 absorb not only a slight displacement of the movable post 13 in the back-and-forth direction due to the application of a load onto the sample scale 20, but also a slight displacement of the point of force 32 in the back-and-forth direction due to the inclination of the lever 30.
In the Roberval's chain 10 having the arrangement above-mentioned, the parallelism between the upper and lower horizontal members 11, 12 is generally important. More specifically, only under the conditions that the upper and lower horizontal members 11, 12 are precisely parallel to each other, the offset position error of a load on the sample scale 20 can be canceled. That is, unless the horizontal members 11, 12 are precisely adjusted such that the distances H, H' in FIG. 9(B) are precisely equal to each other, an offset position error is produced. Particularly, a precise electronic balance of the electromagnetic equilibrium type or the like, requires precision on the order of not greater than .mu.m. Thus, such adjustment is not on the level which can be achieved by measuring the distances H and H'. In an actual adjustment, the parallelism between the horizontal members 11, 12 is to be fine-adjusted such that, when a load is applied to the sample scale 20 at each of a variety of positions thereof, the measured value at each of the positions undergoes no change.
As a mechanism for adjusting the parallelism of the Roberval's chain 10, there is generally known a mechanism as shown in FIG. 11. More specifically, with the use of an adjusting arm 71 having one end resiliently secured to the stationary post 14 and the other end which is free, the vertical movement of the arm 71 by rotating an adjusting screw 72 is reduced to d.sub.2 /d.sub.1, thereby to finely move an attachment portion F of the horizontal member 11 or 12 (e.g., Japanese Utility Model Laid-Open Publication No. 63-308522). Also, there are known a mechanism in which a differential screw is used as the adjusting screw 72 (Japanese Utility Model Laid-Open Publication No. 63-35924), a mechanism in which one horizontal member attachment portion F is finely moved with respect to the stationary post 14 with the use of a wedge principle (Japanese Utility Model Laid-Open Publication No. 62-40531) and the like.
With the recent demand for a smaller and thinner balance, it becomes necessary to increase, as compared with a prior art balance, the lever ratio to measure a large load with a small mechanism. However, if a conventional arrangement is adopted as it is and the lever ratio is simply increased to reduce the balance in size, the offset position error becomes great, failing to satisfy the balance specifications.
More specifically, when the lever ratio is increased to shorten the distance L.sub.1 between the elastic fulcrum 31 and the point of force 32 of the lever 30 to about 1 mm or less, the offset position error particularly in the back-and-forth direction varies dependent on the magnitude of a load. Accordingly, even though the offset position error is precisely adjusted for a certain load using the adjusting mechanism as above-mentioned, there is produced an offset position error for another load. This prevents such a balance from being put to practical use.